An Aqueous Thermodynamic Model for the Complexation of Nickel with EDTA Valid to High Base Concentration

Abstract

An aqueous thermodynamic model is developed which accurately describes the effects of high base concentration on the complexation of Ni2+ by ethylenedinitrilotetraacetic acid (EDTA). The model is primarily developed from an extensive dataset on the solubility of Ni(OH)2(cr) in the presence of EDTA and in the presence and absence of Ca2 + as the competing metal ion. The solubility data for Ni(OH)2(cr) were obtained in solutions ranging in NaOH concentration from 0.01 to 11.6 mol-kg−1, and in Ca2 + concentrations extending to saturation with respect to portlandite, Ca(OH)2. Owing to the inert nature of the Ni-EDTA complexation reactions, solubility experiments were approached from both the oversaturation and undersaturation direction and over time frames extending to 413 days. The final aqueous thermodynamic model is based upon the equations of Pitzer, accurately predicts the observed solubilities to concentrations as high as 11.6 mol-kg−1 NaOH, and is consistent with UV–Vis spectroscopic studies of the complexes in solution.